NANO LETTERS
Controlled Assembly of Magnetic Nanoparticles from Magnetotactic Bacteria Using Microelectromagnets Arrays
2004 Vol. 4, No. 5 995-998
Hakho Lee, Alfreda M. Purdon, Vincent Chu, and Robert M. Westervelt* Department of Physics and DiVision of Engineering and Applied Sciences, HarVard UniVersity, Cambridge, Massachusetts 02138 Received March 19, 2004; Revised Manuscript Received March 31, 2004
ABSTRACT Controlled assembly of magnetic nanoparticles was demonstrated by manipulating magnetotactic bacteria in a fluid with microelectromagnets. Magnetotactic bacteria synthesize a chain of magnetic nanoparticles inside their bodies. Microelectromagnets, consisting of multiple layers of lithographically patterned conductors, generate versatile magnetic fields on micrometer length scales, allowing sophisticated control of magnetotactic bacteria inside a microfluidic chamber. A single bacterium was stably trapped and its orientation was controlled; multiple groups of bacteria were assembled in a fluid. After positioning the bacteria, their cellular membranes were removed by cell lysis, leaving a chain and a ring of magnetic nanoparticles on a substrate.
With the advent of the controlled synthesis of nanocrystals, efforts are being made to assemble these particles into custom-made structures.1-3 By trapping metallic or semiconducting nanoparticles between electrodes, single-electron devices were constructed.4,5 Genetically engineered viruses were used to assemble semiconducting nanocrystals into ordered structures.6,7 The manipulation of magnetic nanoparticles is also of significant interest because single-domain magnets have applications in spintronics, magnetic memory, and biology.8-10 Using either permanent magnets or electromagnets, various magnetic objects, including magnetic nanowires and superparamagnetic particles, were positioned or moved in a fluid.11,12 In this letter, we describe a new approach to assemble magnetic nanoparticles into ordered structures by controlling the motion of magnetotactic bacteria with microelectromagnets. Through highly controlled biomineralization, magnetotactic bacteria synthesize chains of intracellular, singledomain magnetic nanoparticles.13 The cellular bodies enclosing the magnetic chains prevent the magnetic aggregation of the bacteria, making it possible to use the bacteria as a carrier of magnetic nanoparticles. The microelectromagnets create versatile magnetic field patterns on micrometer length scales to guide the motion of magnetotactic bacteria inside a microfluidic chamber.14 After assembling the bacteria with microelectromagnets, the cellular membranes of the bacteria * Corresponding author. E-mail:
[email protected] 10.1021/nl049562x CCC: $27.50 Published on Web 04/08/2004
© 2004 American Chemical Society
Figure 1. Scanning electron micrographs of Magnetospirillum magnetotacticum (MS-1). (a) The bacteria synthesize a chain of magnetite (Fe3O4) nanoparticles, which is anchored inside their body. (b) Close-up of the chain of magnetite nanoparticles. Each particle in the chain is covered with a membrane and has magnetic moment ∼6 × 10-17 A‚m2. The total magnetic moment of a bacterium is ∼1.0 × 10-15 A‚m2.
were removed by cell lysis to leave the biogenic magnetic nanoparticles at desired locations. The novel magnetic structures produced by magnetotactic bacteria have been a subject of active study since their discovery.13,15-17 Magnetotactic bacteria grow magnetic nanoparticles inside their bodies. The mineralization processes are highly regulated by the bacteria, leading to the formation of uniform, species-specific magnetic nanoparticles. Moreover, the particles are assembled into single or multiple chains and anchored inside the cell, enabling the bacteria to passively orient themselves along geomagnetic field lines. Figure 1 shows Magnetospirillum magnetotacticum (MS-1), a variety of magnetotactic bacteria that grow a single chain of intracellular magnetite (Fe3O4) nanoparticles. Each nanoparticle, which is contained in a phospholipid
Figure 2. Schematic and micrographs of microelectromagnets before attaching microfluidic chambers. (a) A ring trap is a circular Au wire topped with an insulating layer. The current I in the wire generates a local magnetic peak that traps magnetic objects on the surface of the device (left). A ring trap of diameter 5 µm was fabricated on a Si/SiO2 substrate (right). (b) A microelectromagnet matrix with electrical leads (left) and its close-up (right). Two layers of 10 Au wires are aligned perpendicular to each other, separated and topped by insulating layers. The width and the pitch of the wires are 5 µm and 10 µm, respectively.
membrane, has a cuboctahedral {111}+{100} crystal structure with a narrow size distribution.15 Furthermore, the diameter of the particle (≈50 nm) falls in a range where the particle is a single-domain permanent magnet with magnetic moment ∼6 × 10-17 A‚m2. Adding up the individual magnetic moments of particles in the chain, the total magnetic moment of a single MS-1 bacterium is ∼1.0 × 10-15 A‚m2. The cellular bodies enclosing the magnetic chain prevent the clustering of bacteria from magnetic dipole interactions. To manipulate and assemble magnetotactic bacteria in a fluid, a micromanipulation system was developed using microelectromagnets and microfluidics. The microelectromagnets generate strong, localized magnetic field peaks to precisely position magnetotactic bacteria in a fluid at room temperature. The microfluidic system controls the fluidic flow, which is crucial for the stable trapping of the bacteria. Figure 2 shows two types of microelectromagnets used in the experiment, a ring trap and a matrix, before microfluidic chambers were attached. The ring trap is a circular conducting wire covered with an insulating layer. The matrix consists of two arrays of straight conducting wires, aligned perpendicular to each other, which are separated and capped with insulting layers. The microelectromagnets were fabricated on Si/SiO2 substrate as previously reported.14 Conducting wires were patterned using either optical lithography or electron beam lithography followed by Cr/Au deposition and lift-off. To reduce the friction between the trapped bacteria 996
and the surface of the device, a resin with good planarization properties was used for the insulating layers. A microfluidic chamber was separately fabricated from poly(dimethylsiloxane)s (PDMS) using soft lithography.18 The width (1 mm) and the depth (100 µm) of the channel were chosen such that the viscous drag on the bacteria by the fluid flow is
